On the magnetic properties of cobalt substituted bovine superoxide dismutase derivatives

We have measured the magnetic susceptibility in the temperature range 1.4-77°K of three derivatives of bovine superoxide dismutase in which Co2+ was substituted for Zn2+: (1) 2Co2+ - in which Co2+ binds to the normal Zn2+ site and the Cu2+ site is unoccupied, (2) 2Co2+2Cu2+ - in which the Zn2+ site is occupied by Co2+ and the copper sites contains Cu2+ and (3) 2Co2+2Cu+ - which is the reduced form of the second derivative. The 2Co2+ protein exhibits Curie paramagnetism indicating S′ = 1 2 and the zero-field splitting must be greater than {similar or greater-than}20 cm-1. The same propeties have been observed with the 2Co2+2Cu+-protein. By contrast, the 2Co2+2Cu2+-derivative exhibits relatively little paramagnetism, some of which arises from non-specifically associated metal ions. The lower susceptibility is due to antiferromagnetic coupling between Co2+ and Cu2+, and the magnitude of the coupling constant is probably ≫5 cm-1. © 1975.